This application claims the priority of Korean Patent Application No. 2002-76212, filed on Dec. 3, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a transceiver for an Asynchronous Transfer Mode Passive Optical Network (ATM-PON), and more particularly, to a transceiver for an Optical Network Unit (ONU), the transceiver including a Laser Diode (LD) as a transmitting device at 1.3 μm and a Photo Diode (PD) as a light receiving device at a 1.55 μm, the LD and the PD being integrated into one chip.
2. Description of the Related Art
In an Asynchronous Transfer Mode Passive Optical Network (ATM-PON) recommended in ITU-T G.983.1, bi-directional communications are carried out for transmitting an upstream signal at 1.3 μm and a downstream signal with a wavelength of 1.55 μm. A conventional transceiver module for bi-directional optical communication includes a transmitting module and a receiving module which respectively transmit and receive a signal through separate optical fiber lines. Recently, a transceiver module in which the transmitting module and the receiving module have been integrated into one package has been developed. However, this transceiver module also transmits and receives a signal through separate optical fiber lines. To reduce the number of optical components and the installation costs thereof, and to thereby implement an economical optical communication network, a transceiver module performing bi-directional communication through a single optical fiber line and a method therefor have been developed. Such a conventional transceiver module comprises a Y-shaped light division waveguide, a Wavelength Division Multiplexing (WDM) filter, a semiconductior laser as a light-emitting device, and an optical detector as a light-receiving device, all these components being integrated in a hybrid manner.
FIG. 1 shows an example of a conventional transceiver module for an ONU, wherein optical devices are integrated in a hybrid manner.
Referring to FIG. 1, an optical signal at a 1.55 μm input through a line of an optical fiber is divided by a Y-shaped light division waveguide 10 and transferred to an optical detector 14 consisting of a PD via a WDM filter 12. Then, the optical detector 14 detects the optical signal. Also, an optical signal at 1.3 μm, output from a LD 22 as a light source, is coupled with the Y-shaped light division waveguide 10, passed through a Y-shaped light division point, and input to the optical fiber 30 via a common waveguide. The WDM filter 12 is mounted to prevent the optical signal at 1.3 μm from being reflected and input to the optical detector 14. In FIG. 1, a reference number “24” represents a monitoring PD.
To fabricate the transceiver module shown in FIG. 1, a light-emitting device, a light-receiving device, a WDM filter, a Y-shaped light division waveguide, etc. are packaged together using a precision optical arrangement method. However, such a fabrication method has a disadvantage, in that insertion loss is large and the outputs of transmitted lights and receiving sensitivities of the received lights are low, since optical couplings using the precision optical arrangement method are required many times between the Y-shaped light division waveguide and the respective optical devices. Furthermore, since the optical devices of the transceiver module are separately fabricated and optically coupled to each other using a precision optical arrangement packaging process, a packaging cost is high, and accordingly, low cost type module cannot be fabricated. In addition, minimization of the transceiver module is difficult.
To overcome these disadvantages, a method to integrate a semiconductor laser, an optical detector, a light division waveguide, etc., on one substrate, in a semiconductor process, has been developed. An integrated device fabricated using this method has an excellent performance, compared to the case where independent devices are used. Also, because the method requires only an arrangement of optical fibers, without other frequent optical arrangements, the packaging process is simplified. However, this method also results in large insertion loss, and the transceiver module is difficult to minimize and integrate into one chip. In addition, the method requires a complicated fabrication process, thereby not allowing a high yield.
FIG. 2 shows another example of a conventional bi-directional transceiver module.
Referring to FIG. 2, a bi-directional optical device 110 is flip-chip bonded on a manual arrangement type substrate 141 such that a light-receiving device 110a faces downward. An optical fiber 120 is arranged and fixed in a V-groove so that an end of the optical fiber 120 is polished inclinedly and an acute portion of the end faces the light-receiving device 110a. The space between the optical device 110 and the optical fiber 120 is filled with a refraction index controlling medium 125 such as silicon gel. A reflection device 133 is mounted behind the monitoring PD 136 of the optical device 110.
In the construction of FIG. 2, a laser as a light source, a monitoring PD, and a light signal detection PD are integrated in a vertical direction. This construction is designed so that a light output from the laser is input to the optical fiber core, while a signal light output from the optical fiber is input to the light receiving device for detecting the signal. This is achieved by polishing the end of the optical fiber into an appropriate angle (35°) according to the Snell's law for an optical fiber to be optically arranged in the integrated optical communication module. According to this method, since the laser and the light-receiving device are integrated in a vertical direction using a semiconductor growth technique, the optical module can be easily fabricated and the precision optical arrangement using the Y-shaped light division waveguide and various optical devices is unnecessary. However, since this method must use a special optical fiber polished to a desired angle and an optical arrangement between an optical fiber and a module is difficult, a packaging cost is high. Also, in a case where a signal light output from the laser is reflected by the optical fiber and input to the light-receiving device, a transmitting light and a receiving light are mixed, and thus, received information can be incorrectly interpreted. Therefore, an output of the transmitting light reflected by the optical fiber must be lower than the receiving sensitivity (−40 dBm) of the light-receiving device. This requires very complicated optical arrangements and precision treatments of the optical fibers, thereby increasing the packaging cost and resulting in a low yield.
FIG. 3 shows a bi-directional transceiver module 200 published in the OFC/IOOC 1999 Technical Digest by Alcatel, as still another example of a conventional bi-directional transceiver module.
The bi-directional transceiver module 200 shown in FIG. 3 includes three parts, i.e., a DFB laser 210 at a 1.3 μm, an absorber 220 at 1.3 μm, and a light-receiving device 230 at 1.55 μm. Referring to FIG. 3, a receiving light signal of 1.55 μm as a downstream signal is not absorbed by the 1.3 μm DFB laser 210 and the 1.3 μm absorber 220 and is detected by the light receiving device 230. A transmitting signal from the 1.3 μm DFB laser 210 is transmitted in a upward direction through the optical fiber, and an optical signal output from the rear-end of the 1.3 μm DFB laser 210 is absorbed by the 1.3 μm absorber 220. However, the absorber 220 at the center portion must completely absorb the transmitting signal from the 1.3 μm DFB laser 210 so that the output power of the transmitting signal is lower than the receiving sensitivity (−40 dBm) of the light-receiving device 230 as an optical detector, in order to divide the optical transmitting signal from the optical receiving signal and correctly transfer received information. However, it is very difficult for the absorber 220 to completely absorb a transmitting light with a very high output. Accordingly, the structure described above is very difficult to be implemented in spite of its simple arrangement.
As described above, to reduce installation costs of optical fibers and the number of optical components needed for optical communication, research on a bi-directional transceiver module that transmits and receives a signal on one line of optical fiber have been carried out. However, a proposed technique for integrating various optical components in a hybrid manner increases the size of the resultant transceiver module, increases insertion loss, requires a precision optical arrangement, has low economy, and results in a low yield. Also, single integration techniques proposed heretofore are still problematic in view of dividing an optical transmitting signal from an optical receiving signal and correctly obtaining received information.